幻想郷の住人たちが現代日本に現れたのか、
交通標識が幻想郷に流れ着き出現してしまったのか
何れもあり得るだろう

Either the inhabitants of Gensokyo have appeared in modern-day Japan,
Or a traffic sign has drifted into Gensokyo and appeared there,
Either possibility is possible.